This week, the question is: “Why do Clojurians make such a big deal about immutability?” We cover several practical side effects of immutability and why we’ve become such big fans of data that doesn’t let us down.

Selected quotes:

“Well, we don’t have Monads to talk about.”

“What good is a program, if you can’t change stuff!?”

“The memory cost of data structures is in proportion to the changes, not the users.”

“I can hang on to a reference to the old state and a reference to the new state very cheaply.”

“You can reduce comparison to referential equality.”

“Once you can efficiently save every version of the state, going back to a previous version is no big deal.”

“I love determinism. Determinism is trustable.”

“I can trust immutable data. And if I can trust it, then it can occupy a smaller part of my brain.”